1. Field of the Invention
The present invention relates to a variable valve timing system in an engine having a rotating cam-shaft, and more particularly to a variable valve timing system in an engine having a rotating camshaft for driving intake and exhaust valves.
2. Discussion of the Background
A conventional variable valve timing system is disclosed in Japanese Patent Publication Laid-open Publication No. 62(1987)-3111 published without examination, and is shown in FIG. 4. The variable valve timing system 500 is used for an engine of a vehicle (not shown). In the variable valve timing system 500, a timing pulley 501 has an inner helical gear 501a and an outer gear 501b. The outer gear 501b is geared with a timing belt 502. A cam-shaft 503 is rotatably supported in a cylinder-head 506 of the engine and has an oil conduit 503a formed therein. A cylindrical member 504 forms an outer helical gear 504a and is held on the cam-shaft 503 by a hollow bolt 507.
A cylindrical piston system 505 includes a first piston 505a, a second piston 505b, a plate 505c, a first spring 505d and a second spring 505e. The first piston 505a and the second piston 505b have inner helical gears 505a-1 and 505b-1 and outer helical gears 505a-2 and 505b-2, respectively. The inner helical gears 505a-1 and 505b-1 are geared with the outer helical gear 504a. The outer helical gears 505a-2 and 505b-2 are geared with the inner helical gear 501a.
The first piston 505a is operatively connected with the plate 505c. The first spring 505d is interposed between the first piston 505a and the second piston 505b, so that the first piston 505a, the second piston 505b and the first spring 505d constitute a scissors gear system for decreasing backlash.
A cam-shaft cover 508 is fixed to the timing pulley 501 by bolts 509. A pressure chamber 510 is formed between the plate 505c and the cam-shaft cover 508. The pressure chamber 510 is in fluid communication with the oil conduit 503a via the hollow bolt 507.
In the above-mentioned variable valve timing system 500, the timing belt 502 is driven by a crank-shaft of the engine (not shown). Thus, the timing pulley 501 is rotated by the timing belt 502, and the cam-shaft 503 is rotated through the cylindrical piston system 505. The cam-shaft 503 drives some intake and exhaust valves of the engine (not shown) so that those intake and exhaust valves are opened or closed. A change of the revolution speed of the engine requires a change in the timing by which these valves are opened or closed.
High-pressure oil supplied from an oil tank (not shown) through a control valve (not shown) and the oil conduit 503a is introduced into the pressure chamber 510. This causes the cylindrical piston system 505 to move in the rightward direction. Therefore, the relative angle between the timing pulley 501 and the cam-shaft 503 is changed via the helical gears 501a, 505a-2 and 505b-2 located between the timing pulley 501 and the cylindrical piston system 505 and by the helical gears 505a-1, 505b-1 and 504a located between the cylindrical piston system 505 and the cam-shaft 503. Consequently, the timing by which the intake and exhaust valves are opened or closed is changed.
In the normal driving of the engine, the cylindrical piston system 505 receives a torque variation from the cam-shaft 503 as the cam lobes sequentially engage and disengage the cam followers, producing rotational vibrations between the timing pulley and the cam-shaft. As a result, the cylindrical piston system 505 may move in the rightward direction, even though the high pressure oil is not supplied to the pressure chamber 510. Thus, the spring 505e must be strong to avoid such movement of the cylindrical piston system 505.